Conventionally, in color image pickup apparatus of this kind, an image pickup device (a CCD or the like) provided with a plurality of color separation filters, for example stripe filters, having different spectroscopic characteristics is used.
For example, the case is considered where a red light (R) transmission filter, green light (G) transmission filter, and blue light (B) transmission filter (hereinafter, simple called R filter, G filter and B filter) are sequentially arranged on the surface of the image pickup device as shown in FIG. 1. The incident rays transmitted through such filters and an optical system to the image pickup device are spatially sampled by the above-mentioned color stripe filters and image pickup device. However, in this case, for the spatial sampling frequency 3fc (fc being the spatial sampling frequency for one of the plurality of colors) to be determined on the basis of the number of picture elements of the image pickup device or the pitch of the color stripe filter and the readout speed of the signal of each picture element, the spatial frequency component of the incident rays which are equivalent to not lower than 1/2 of this 3fc becomes a cause of the fold distortion (also called frequency aliasing noise). This point will be described hereinbelow with reference to FIGS. 2A to 2C. In FIGS. 2A to 2C, the abscissa indicates the frequency and the ordinate represents the signal level.
The incident rays sampled on the image pickup device are readout as an image pickup signal from the image pickup device due to the photoelectric conversion action thereof, or the like. Now, when attention is paid to only the image signal of R (or only G or B), its repetitive frequency is 1/3 of the readout frequency 3fc. If this repetitive frequency is fc, the base band component and side band component of the incident rays due to the sampling are as shown in FIG. 2A, and the hatched section is called a fold distortion component. When this signal passes through a low-pass filter having such a characteristic as shown in FIG. 2B, this fold component is mixed with the base band component and remains, so that this component will cause the picture quality on a display remarkably to deteriorate. A method of reducing such fold distortion is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 56-120281. That is to say, as shown in FIG. 2C, when an achromatic color object image is pictured, if the transmission characteristics of the color separation filter are designed in such a manner that the levels of the dot-sequential signals to be output from the image pickup device are 1:1:1, the vectors of the side band components will be cancelled mutually, so that this enables the fold distortion to be reduced.
According to this method described above, it is possible to reduce the fold distortion with respect to at least an achromatic color screen.
Of course, such an effect cannot be obtained with respect to the screen with high color saturation. However, this point may be ignored since the luminous efficiency characteristic (or factor) of the human eye does not correspond to the spectral sensitivity characteristic of the image pickup device and is lower with respect to the color on the higher frequency side.
However in the above-described case, when the color temperature of a light source for picturing differs in dependence upon the objects or locations of picturing, the dot-sequential signal levels become unbalanced, so that the fold distortion would have eventually occurred. FIG. 3 shows the spectroscopic energies at color temperatures of, e.g. 3200.degree. K. and 6000.degree. K. FIG. 4 is a diagram showing the drawback in the case where the color separation filter was designed such the the dot-sequential output signal levels become constant at, e.g., 3200.degree. K. Even if the filter was set such that the color temperature levels of R, G and B become 1:1:1 at 3200.degree. K., the level on the longer wavelength side, i.e. on the R side will have become weak and that on the B side will, on the contrary, have become strong at 6000.degree. K. as shown in FIG. 4. Thus, the vectors of the side band will have been deflected on the side of cyan (Cy) as shown in FIG. 4, causing the fold distortion to occur.
Furthermore, since the image pickup device generally has a high sensitivity for infrared rays and there is a discrepancy with respect to the luminous efficiency between the human eye and the image pickup device, a filter to cut the infrared rays is provided in the incident light path to the image pickup device to prevent this drawback. However, variation in frequency characteristic of this kind of filter occurs due to the variation in thickness caused in the manufacturing process thereof. As a result of this fact, there may be a case where the R signal level will change.
As a method of eliminating such a drawback as described above, the following two methods are principally known.
One method is to utilize a mechanical color temperature compensation filter. With this method color temperature compensation filters for, e.g., daylight, fluorescent lamp and tungsten are generally mainly prepared and they are changed over in accordance with the imaging locations. This method has drawbacks such that several kinds of compensation filters are needed and that it is impossible to completely prevent the occurrence of fold distortion due to imperfect level adjustment of the dot-sequential signal levels.
The other method is to provide means for multiplying the control signals in such a manner that the dot-sequential signal levels of R, G and B become 1:1:1 as set forth in Japanese Unexamined Patent Publication (Kokai) No. 57-26977. According to this method it is possible in principle to accurately match the signal levels, but there is a drawback such that the circuit becomes extremely complicated since the control signal multiplication means is driven at high frequency. That is, when the sampling frequency of each color signal is about 3.8 MHz, respectively, the image pickup device has to be driven at about 11 MHz in order to obtain the three color signals, but in this case, the dot-sequential pulses to form the luminance signals also become about 11 MHz.
In addition, to perform the dot-sequence processing with the timing with a certain degree of accuracy, the leading edge of the dot-sequential pulse must have the component of about 30 MHz, which is approximately three times that of the dot-sequential pulse. However, there is a drawback such that it is extremely difficult to simultaneously correctly adjust the levels in such a high frequency region.